High speed transmission of data in a railroad environment presents problems which are difficult to solve. The rate of communication failures (undetected error or lack of communication) must be very low so as to be compatible with safety requirements. The use of the available material carriers, such as power transmission lines (overhead lines or surface lines) does not allow sufficiently high data transmission rates to be attained for installations achieving automatic control and automatic separation and regrouping of the vehicles. The use of transmission channels without a material carrier seems at first sight to be excluded, due to the very variable character of the environment and to the risks of reflection, particularly on the walls of cuttings or tunnels and on vehicles moving over a parallel track.
Beams of ultra sounds, which have however the advantage of being very directional, have proved inappropriate. The use of hertzian waves, in the microwave range, also seems to be excluded due to possible reflection on fixed or moving obstacles and to the risk of collision, in the receiver, between messages actually intended for this receiver and other messages.